Garage Supply - Earth Question

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sjwkmje

Hi,

I recently wired a garage that is 4 meters from the house. I split the tailes and fed a seperate CU with a single RCBO (63a). This connects to 6mm twin and earth up and accross the loft and down the side of the house in galv ducting. This then goes to an IP66 diecast box which terminates the SWA at the side of the house. The SWA is 6mm. This runs under ground to the garage and comes up into a crabtree metal clad box.

I want to know if the garage can rely on the eath that is used with the house. When I split the tails I ran 16mm earth to the CU and connected the earth from the twin and earth there. The earth on the twin and earth connects to the earth in the SWA (3 core and earth) and also to the sheeth. The galv ducting as it runs high on the house has a dedicated earth spike with external clamp into the pipe work as a low resistance path if it gets hit by lighhting.

Now at the garage end do I need a earth spike? If I fit one do I connect the earths from the SWA with the one from the earth spike in side the CU on the earth bar. Does the earth spike then come suplementry bonding? Do I need the spike?

Been thinking about this for some time and nothing in the regs seem to help.

I know that the Circuit Earth Loop Impedance value (Zs) must conform to stated values within the Guidance Notes 3 IEE Inspection and Testing of Electrical Installations to ensure safe operation of protective devices within the correct disconnection times.

Adam
 
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As I understand it there is nothing wrong with using the house earth to earth equipment in the garage. However if the supply is PME and there is metal pipework in the garage that needs main bonding then the earth to the garage needs to be up to PME main bonding standards.
 
Are you saying that tou split the tails from the meter which imples that there is no double pole isolator between the meter and the RCBO?

If it is a single module RCBO it does not break the neutral so this does not comply with the 7671 requirement for an isolator to break all live conductors (neutral is a live conductor).
 
You are, of course, also aware that 6.0mmsq T+E is not rated to 63A, least not in conduit.

As already stated, the independant CU for the garage needs to have a DP isolator. You need to decrease the rating of your protective device, not taking voltage drops into account a quick guess would say 32A is a more suitable value.

Furthermore, I would remove the earth spike you have installed for the conduit and earth it via the CPC of the 6mmsq T+E or via singles back to the earth bar in the CU. It isn't going to get hit by lightning, I'm pretty sure your chimney or the apex of the house is higher. What's more, if a fault ever develops with the cable and a live wire comes into contact with the conduit/ducting then your earth spike doesn't guarantee a low resistance path back to earth.

If you make the above changes then I can see no reason for an earth spike at the garage, although you haven't told us what sort of supply you're on.
 
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Are you saying that tou split the tails from the meter which imples that there is no double pole isolator between the meter and the RCBO?

If it is a single module RCBO it does not break the neutral so this does not comply with the 7671 requirement for an isolator to break all live conductors (neutral is a live conductor).

Sorry I split the tailes ran an 100A isolator (switch fuse) then to a double pole RCBO so that it breaks live and nerual and complys with 7671 requirements for isolation.

The house supply is a PME system. I will remove the spike.

Adam
 

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